97 | | |
98 | | = 1 - Objectives = |
99 | | |
100 | | Our primary objective is to measure detailed turbulence distributions within channelised gravity currents, as a function of Coriolis forces, concentrating on: i) the bottom boundary layer, ii) redistribution of turbulence within bends, and, iii) redistribution of turbulence at the interface between the gravity current and the ambient. These datasets will enable existing theory on the presence and influence of Ekman boundary layers to be tested, with important implication for the basal shear stress distributions, erosion, and the evolution of channels. These data on the distribution of turbulence will then be applied to i) examine the turbulence distribution in straight channels, ii) provide an analysis of secondary flow and associated turbulence around bends for the first time, and an assessment of how channelized flows alter as a function of Rossby numbers and therefore latitude, iii) assess how the morphodynamics of submarine channels vary as a function of the Rossby number, iv) explain the observed patterns of submarine channel sinuosity with latitude (Peakall et al., 2012; Cossu and Wells, 2013; Cossu et al., 2015), and, v) incorporate the entrainment data into numerical models of submarine channels, in order to address the unanswered question of how these flows traverse such large-distances across very low-angle slopes (Dorrell et al., 2014).Our primary objective is to measure detailed turbulence distributions within channelised gravity currents, as a function of Coriolis forces, concentrating on: i) the bottom boundary layer, ii) redistribution of turbulence within bends, and, iii) redistribution of turbulence at the interface between the gravity current and the ambient. These datasets will enable existing theory on the presence and influence of Ekman boundary layers to be tested, with important implication for the basal shear stress distributions, erosion, and the evolution of channels. These data on the distribution of turbulence will then be applied to i) examine the turbulence distribution in straight channels, ii) provide an analysis of secondary flow and associated turbulence around bends for the first time, and an assessment of how channelized flows alter as a function of Rossby numbers and therefore latitude, iii) assess how the morphodynamics of submarine channels vary as a function of the Rossby number, iv) explain the observed patterns of submarine channel sinuosity with latitude (Peakall et al., 2012; Cossu and Wells, 2013; Cossu et al., 2015), and, v) incorporate the entrainment data into numerical models of submarine channels, in order to address the unanswered question of how these flows traverse such large-distances across very low-angle slopes (Dorrell et al., 2014). |
101 | | |
102 | | |
103 | | = 2 - Experimental setup: = |
104 | | |
105 | | [[Image(Set_up_drawing.jpg)]] |
106 | | |
107 | | |
108 | | |
109 | | == 2.1 General description == |
110 | | |
111 | | 2.1 General description |
112 | | A channel model is positioned within the Coriolis facility. The channel model consists of an initial tapered input section with a honeycomb baffle for flow straightening and turbulence control, a 3.2 m straight channel section, and two bends with a mid-channel radius of 1.5 m. The channel is made of acrylic and is 60 cm wide and 50 cm high; the sinuous section has a sinuosity of 1.2. The slope is 1/50 radians (2% gradient) and the channel terminates 10 cm off of the floor. Saline fluid is pumped into the top of the channel, forming a gravity current, which flows along the channel, and off the end. The basal 10 cm of the flume operates as a sump for the denser saline fluid to accumulate. In turn, this fluid can be drawn down in one of two ways: i) whilst recirculating the fluid, though this is limited to 20 m3/hr (5.55 l/s), and ii) through emptying to the drain, in which case any flow rate is possible. |
113 | | Two long metal rails are positioned to either side of the channel model across the full width of the flume. These carry a computerized gantry, which can be positioned at any point along the channel. The gantry itself contains the controls for two Schneider slides, one orientated transverse to the model, and the other connected slide, orientated in the vertical. Thus the system enables xyz control. |
114 | | |
115 | | |
116 | | == 2.2 Definition of the co-ordinate system == |
117 | | |
118 | | |
119 | | == 2.3 Fixed Parameters == |
120 | | |
121 | | ||'''Notation'''||'''Definition'''||'''Values'''||'''Remarks'''|| |
122 | | || $Q_0$ || Input Density || $12 \ ls^-^1$ || || |
123 | | || $\Delta\rho$ || Density Difference || $20 \ kg \ m^-^3$ || || |
124 | | || $W$ || Channel Width || $0.6 \ m$ || || |
125 | | || $\nu$ || Viscosity || $10^-^6m^2s^-^1$ || || |
126 | | || $S$ || Slope|| $3.5^{\circ}$ || || |
127 | | |
128 | | |
129 | | == 2.4 Variable Parameters == |
130 | | |
131 | | ||'''Notation'''||'''Definition'''||'''Unit'''||'''Initial Estimated Values'''||'''Remarks'''|| |
132 | | || $\Omega$ || Rotation Rate || $rads^-^1$ || -0.18 - 0.15 || || |
133 | | || $H_w$ || Water Depth || $m$ || 1-1.1 || || |
134 | | || $Q_o_u_t_p_u_t$ || Output Flow Rate || $ls^-^1$ || 5.5 - 17 || || |
135 | | || $k$ || Roughness || - || - || || |
136 | | |
137 | | |
138 | | == 2.5 Additional Parameters == |
139 | | |
140 | | ||'''Notation'''||'''Definition'''||'''Unit'''||'''Initial Estimated Values'''|| |
141 | | || $h$ || Depth of gravity current || $m$ || 0.1 || |
142 | | || $U$ || Mean downslope velocity || $m^s^-^1$ || 0.1-0.15 || |
143 | | || $\delta$ || Thickness of Ekman boundary layer || $mm$ || ~10 || |
144 | | || $R$ || Radius of curvature || $m$ || 1.5 || |
145 | | |
146 | | == 2.6 Definition of the relevant non-dimensional numbers == |
147 | | |
148 | | |
149 | | Flow Reynolds number across the obstruction, $Re = Uh/\nu$. |
150 | | |
151 | | Densimetric Froude number, $Fr = U/(g'h)^{1/2}$, $g' = g(\Delta\rho)/\rho_0$. |
152 | | |
153 | | Rossby number, $Ro = U/fW$. |
154 | | |
155 | | Canyon number, $\beta = sW/\delta$. |
156 | | |
157 | | |
158 | | = 6 - Table of Experiments: = |
159 | | |
160 | | ||'''Run Name'''||'''Measurement Position'''||'''Density Excess '''||'''Input flow'''||'''Rotation Rate'''||'''Initial Water Depth'''||'''Outflow Rate'''||'''Run Time'''||'''ADV Dwell Time'''|| |
161 | | || || || $(kg \ m^-^3)$ || $(ls^-^1)$ || $(rad \ s^-^1)$ || $(m)$ || $(ls^-^1)$ || $(Minutes)$ || $(s)$ || |
162 | | ||Smooth_1|| 1 || 20 || 12 || 0 || 1 || 5.5 || 20 || 60 || |
163 | | ||Smooth_2|| 1 || 20 || 12 || 0 || 1 || 12.0 || 20 || 60 || |
164 | | ||Smooth_3|| 2 || 20 || 12 || 0 || 1 || 5.5 || 20 || 60 || |
165 | | ||Smooth_4|| 3 || 20 || 12 || 0 || 1 || 5.5 || 20 || 60 || |
166 | | ||Smooth_5|| 4 || 20 || 12 || 0 || 1 || 5.5 || 20 || 60 || |
167 | | ||Smooth_6|| 1 || 20 || 12 || 0 || 1 || 5.5 || 20 || 30 || |
168 | | ||Smooth_7|| 3 || 20 || 12 || 0 || 1 || 5.5 || 20 || 30 || |
169 | | ||Smooth_8|| 4 || 20 || 12 || 0 || 1 || 5.5 || 20 || 30 || |
170 | | ||Smooth_9|| 2 || 20 || 12 || 0 || 1 || 12.0 || 60 || 30 || |